This invention relates generally to the field of hollow tubing for use in various oil and gas well applications. In particular, the invention relates to non-metallic tubing and especially to small diameter or thin-walled tubing.
Hollow tubing is used in various applications in the recovery of oil, gas, water and other liquids and gases from underground geological formations through "wells." For example, tubing is used as "casing" to form an exterior protection against collapse of the drilled hole. It is used as "production tubing" to convey the liquids or gases from the zone or zones from which they are extracted to the surface. Tubing is used in a variety of other oil well applications, such as, the connection of various machinery to surface or underground pumps.
Despite the long history of underground extraction of liquids and gases through wells, the predominant material utilized as tubing in such applications has been and still is steel or steel alloys. Although steel is expensive relative to other materials, it is commonly utilized because of its structural characteristics. Steel tubing is formed into sections that are threaded at each end to form secure connections. The threaded sections form a tube or "string" that may extend vertically underground for more than a mile placing severe tensile forces on the upper sections of the tubing. The lower sections of the tubing are subjected to horizontal compressive forces resulting from the elevated temperatures and pressures inherent in the subterranean environment. Steel tubing is generally capable of withstanding those stresses.
Nevertheless, steel tubing does have certain deficiencies. It corrodes. It can be very expensive. It is heavy, requiring massive surface equipment to install, support and maintain the tubing. The process of installing steel tubing in segments is laborious and time consuming. Flexible, continuous, i.e., "coiled," steel tubing is available in thin walled sizes for use in certain installations, such as those requiring a narrow diameter pipe. However, coiled steel tubing is extremely expensive. In at least some applications, i.e., where production is marginal, installation of coiled steel tubing is prohibitively expensive. In addition, steel is less effective than other materials, because it has a higher coefficient of friction, thereby requiring added force to produce oil and gas from subterranean depths through a long length of pipe.
For these reasons, there has been an interest in developing tubing made of materials other than steel for oil field applications. At one time, it was believed that plastics and other non-metallic materials could not be used, because they could not withstand the pressures, temperatures and/or tensile loads presented in bottom hole environments. Efforts to utilize non-metallic pipe in oil field applications have been reported in the patent literature. See, for example, U.S. Pat. Nos. 3,878,312, 4,024,913, 4,349,050, 4,516,608, 4,893,658, 4,999,389, 5,179,140, and 5,213,379, the more recent of which refer to the use of filled resin or fiber reinforced plastic pipe. These products have not been widely used in oil well applications and appear to suffer from several significant defects. For example, most of the materials are highly elastic. A number of the patents listed above attempt to address problems of cracking and deterioration of the threaded joints connecting the pipe sections. In addition, the materials by themselves do not provide substantial tensile strength to withstand the loads placed on the pipe by a long vertical pipe string. Although the substitute materials are generally lighter than steel, a long vertical column of tubing still presents a substantial weight stress, which upper segments of tubing are not capable of resisting. As a result, cracking and failure of the tubing will occur particularly upon any attempt to "pull," i.e., remove, the tubing from the well. These two major problems are not unrelated; the lack of tensile strength is consistent with high elasticity and a lack of structural integrity. Regardless of the reason, to the best of applicant's knowledge no economically viable alternative to steel tubing has been widely accepted for use in oil wells.
The invention presents a solution to the use of non-metallic tubing for oil well applications. As used herein, "non-metallic" refers to tubing that is not made of solid metal. However, the term can include plastics and other materials in which metal fragments, chips or fibers may be used as a filler or reinforcing agent.